This document discusses an artificial immune system approach for detecting computer viruses. It begins by providing background on artificial immune systems and how they can be applied to computer security similar to how the human immune system distinguishes self from non-self. It then describes the proposed artificial immune system-based virus detection system, which includes a signature extractor that generates signatures for non-self programs that do not match self programs, and a signature selector that analyzes the signatures to determine if they belong to viruses or self programs. The system aims to detect unknown viruses through an adaptive process of learning virus signatures.

1. M. Bubak et al. (Eds.): ICCS 2004, LNCS 3037, pp. 499–506, 2004.
© Springer-Verlag Berlin Heidelberg 2004
Artificial Immune System against Viral Attack
Hyungjoon Lee
1
, Wonil Kim
2*
, and Manpyo Hong
1
1
Digital Vaccine Lab, G,raduated School of Information and Communication
Ajou University, Suwon, Republic of Korea
{prime, mphong}@ajou.ac.kr
2
College of Electronics and Information Engineering,
Sejong University, Seoul, Republic of Korea
wikim@sejong.ac.kr
Abstract. Since the first computer virus has been found, scanning detection has
been used as a primarily method in virus detection systems. As computer
viruses and worms become more complex and sophisticated, the scanning
detection method is no longer able to detect various forms of viruses and worms
effectively. Many anti-virus researchers proposed various detection methods
including artificial immune system to cope with these characteristics of viruses
and worms. This paper discusses some principle of artificial immune system
and proposes artificial immune based virus detection system that can detect
unknown viruses.
1 Introduction
Since the computer virus first appeared in 1981, it has been evolved as computer
environments such as operating system, network, and application program have
changed. The frequent evolution of computer virus has forced anti-virus researchers
continuous development of new detection method. However most anti-virus systems
are still based on scanning detection using signature, since other mechanisms have
false positive or detection speed problem in real situations. In this paper, we propose a
new virus detection approach that employs the artificial immune system. Artificial
immune system is based on human immune system. The human immune system
recognizes, memorizes, and responses to virus adaptively. Anomaly detection and
adaptability are important properties of artificial immune system. The proposed
artificial immune based system applies this property and detects unknown computer
viruses.
In the next section, Artificial Immune system will be discussed in detail. In Section
3, the proposed artificial immune based virus detection system will be described.
Simulation results and the direction of future research will be discussed in Section 4
and Section 5 respectively.
*
Author for correspondence +82-2-3408-3795

2. 500 H. Lee, W. Kim, and M. Hong
2 Artificial Immune System
Immunologists have traditionally described the human immune system as the problem
of distinguishing "self" from dangerous "non-self" and eliminating dangerous non-
self. Similarly, the computer security can be viewed as the problem of distinguishing
benign program from malicious program such as computer virus or worm. Spafford
described a computer virus from a biological point of view by analyzing several
characteristic of virus as in artificial life [1]. Since then, many anti-virus researchers
have researched on artificial immune system to protect a system from the intrusion
including viruses and worms.
Artificial immune system should have anomaly detection capability to defend
unknown intrusion. Adaptability is also a necessary property of artificial immune
system to learn unknown intrusion and to response learned intrusions quickly. Other
properties such as distributability, autonomy, diversity and disposability are required
for the flexibility and stability of artificial immune system [14].
Protecting static File Protecting Process Protecting a Network
M atching the system call sequence
using negative selection
Decoy program
self, non-self discrimination
Kill-signal NAVEX(Symantec)
Distributed CVIS
Artificial Immune System
Fig. 1. Taxonomy of artificial immune system
Artificial immune system can be divided into three categories according to which
computer component will be identified as a protected cell. Figure 1 shows this
classification.
First, protecting static file category assumes a file including program code as a cell.
Forrest proposed a method for change detection using negative selection algorithm
[6]. Forrest’s artificial immune system randomly generates check code called
detector, and eliminates ones that can recognize self (benign program). This process is
called negative selection algorithm. All programs or data matched with detectors are
indicated as non-self (malicious code). Kephart proposed an artificial immune model
that consists of monitoring, scanning and biological immunology [11]. In his model,
known viruses are detected by scanner based on weak signature, and unknown viruses
are collected and analyzed using decoy program. The decoy program is a dummy
program that is not changeable. If decoy program is modified, it is virus infection.
Distributed Computer Virus Immune System (CVIS) combined negative selection and
decoy program. It uses decoy program to detect the unknown virus and employs
negative selection to identify detected unknown virus [16].
Second, protecting process category assumes a process as a cell. Matching the
system call sequence is proposed by Forrest [10]. The mechanism of this system is the
same as self-nonself discrimination using negative selection. It is different from the
first category that this mechanism matches detector with system call sequences, not
with program codes.

3. Artificial Immune System against Viral Attack 501
Third, protecting a network category assumes a host as a cell. Kill-signal
mechanism is proposed by Kephart, and a immune system called NAVEX that is
proposed by Symantec [11,17]. Both immune systems propagate signatures of
detected viruses to protect network connected hosts, as does biological immune
system. In biological immune system, when an immune cell detects a virus, the cell
spread chemical signal in order to duplicate the same immune cells.
SelfN on-self
Sig natures Sign atu res Sig natures
Signature Selecto r
V iru s Scanning Signatures
Self
sign atu res
program
program
program
Sign atu re E xtractor
Fig. 2. Structure of proposed virus detection system
3 Proposed Virus Detection System
Negative selection algorithm and decoy programs are important methods in artificial
immune system. Negative selection algorithm is inspired by genetic random-process.
But this mechanism has a weakness that come from the difference between human
body and computer system. State of human body is stable, whereas that of computer
system is not. Negative selection algorithm is suitable in stable system. Idea of decoy
program comes from the distributability and diversity of immune system. In the case
of decoy program, there is no guarantee that a virus will attack a decoy program. The
proposed artificial immune based Virus Detection System (VDS) compromises this
weakness, and is suitable for dynamically changing environment such as computer
system.
VDS is a signature learning system to detect unknown viruses. VDS classify
incoming or changed (patched) programs into legitimate programs and viruses. The
process of VDS is consists of the following three steps. In the first step, VDS assumes
that all existing programs are legitimate. In the next step, all incoming and changed
programs are classified into suspicious programs. Finally, VDS selects virus programs
from these suspicious programs using detection method based on virus behavior.
Hereafter, we refer to legitimate program as self and suspicious program as non-self.
VDS consists of signature extractor and signature selector as in Figure 2. Signature
extractor produces signatures of non-self. The main operation of signature extractor is
selecting bit strings that are not matched with any self code at the same position.
Therefore, signature extractor produces signature of non-self that never matched with
any self. Produced non-self signatures are collected and analyzed by signature
selector.
Signature selector compares the similarity of non-self signature with each others.
Since viruses tend to infect other programs, if the same non-self signatures are
appeared frequently, signature selector classifies those into signatures of viruses, and
these signatures are used for virus scanner. Notice that these non-self signatures never

4. 502 H. Lee, W. Kim, and M. Hong
match with any self program. In the case of less frequent signatures, it is assumed as
signature of self. We classify the program that contains any one of these self
signatures as self. This process provides the adaptable capacity to the proposed VDS.
The detailed process of each steps are discussed in the following 3 sections.
3.1 Signature Representation
The VDS generates signatures to recognize self and non-self program. The size of
signature extraction region, from which we analyze for signature extraction, is always
constant. Since most infected file executes virus code at first, most entry pointer of
infected program indicates virus code. Therefore, we decide that starting point of
signature extraction region is entry point of program. Signature consists of several
pairs of offset and bit string. Number of pairs, offset and bit string are selected by
signature extractor.
Signature
extraction
region
String1
String2
String3
String1
Signature
offset offset String2 offset String3
Progrram
program
entry
point
Fig. 3. Signature representation
3.2 Signature Extractor
Signature extractor produces non-self program signature that never matches with any
self-program signature. This process consists of the following steps.
00100101 00110101 00100101 00100100 ......
self1
non-self
00100101 00111101 11100101 00110101 ......
mark
XXXXXXXX 00110101 00100101 00110101 ......
00100101 00111101 11100101 00110101 ......
self2
mark
XXXXXXXX 00110101 00100101 XXXXXXXX ......
signature
updated
non-self
...
...
Fig. 4. Generating signature of non-self program
• Step 1: Divide a signature extraction region into several same sized comparison
units.
• Step 2: Compare a signature extraction region of non-self program with each one
of all self-programs.

5. Artificial Immune System against Viral Attack 503
• Step 3: If two comparison units on same position are same, mark the comparison
unit in signature extraction region of non-self program. Continue this process on
each signature extraction region of all self-programs.
In Figure 4, comparison unit is 1 byte. Non-self is compared with self1 and self2.
When two comparisons are finished, first unit and fourth unit are marked. After all the
comparisons with other self-programs are done, the signature of non-self consists of
remained unmatched units. Since all of the comparison units that are the same with
self’s are marked as useless, the generated signature never matches with any self-
programs.
3.3 Signature Selector
After we obtain signatures of non-self programs, we need to classify non-self program
into virus or normal program. If virus infects other programs, the signature extractor
generates signatures from same virus code because infected program is changed by
one virus. Therefore, checking frequency of occurrence of the same signature is the
same as checking the spread of the same virus code.
10110001 XXXXXXXX 00100101 XXXXXXXX 10110001 10000001
XXXXXXXX 00110101 00100101 XXXXXXXX 10110001 10000001signature1
signature2
Similarity value = 1 + 11 = 1 + 3 = 4
Fig. 5. Similarity between two signatures
Signature selector calculates the similarity values of non-self signatures, as shown
in Figure 5. Comparison factors are bit sequence and offset of comparison unit in
signature extraction region. If two factors of comparison units are equal, similarity
function adds one to similarity value. When consecutive comparison units are equal,
similarity value increases. For example, if two compare units are equal and adjacent,
similarity value is 3 (112
). In other words, when n consecutive comparison units are
equal, similarity value is 2
n
-1.
Note that signatures of the same programs are more similar than signatures of
different programs. In other words, similarity values between signatures of the same
program codes are higher than the others. Therefore, VDS can distinguish signatures
of the same program codes from signatures of other distinct program codes. The
threshold values for classifying signatures of the same and different programs are
determined by analyzing similarity values of the entire non-self programs.
Determining the threshold values for classifying is discussed in Section 4.
4 Simulation
VDS extracts signatures of non-self programs based on self-programs. Signature size
and similarity value are important factors in VDS. As the number of self-programs
that has the same comparison units at the same position is increased, the size of a

6. 504 H. Lee, W. Kim, and M. Hong
signature of the non-self program is decreased. In the worst case, VDS cannot
generate the signature of particular program, because every comparison unit is
marked as useless. The similarity value is used for classifying virus detector from
suspicious signatures. In this section, we will discuss several simulation results from
various signature extraction region size and comparison unit. The simulation is
processed on several parameters as in Table. 1.
Table 1. Simulation parameters
Parameters Variables
The number of self-programs 1385 execution files
The number of non-self programs 160 execution files (3 virus infected files)
Signature extraction region size 500byte, 1Kbyte, 5Kbyte, 10Kbyte
Comparison unit size 1byte, 2byte, 3byte
(a) SER 10Kbyte
0
2000
4000
6000
8000
10000
12000
0 50 100 150
Files
SignatureSize(byte)
1byte 2byte 3byte
(b) SER 5Kbyte
0
1000
2000
3000
4000
5000
6000
0 50 100 150
Files
SignatureSize(byte)
1byte 2byte 3byte
(c) SER 1Kbyte
0
200
400
600
800
1000
1200
0 50 100 150
Files
SignatureSize(byte)
1byte 2byte 3byte
(d) SER 500byte
0
100
200
300
400
500
600
0 50 100 150
Files
SignatureSize(byte)
1byte 2byte 3byte
Fig. 6. Simulating using various sizes of SER: Graphs show signature size of different
Signature Extraction Region (SER) 10Kbyte; 5Kbyte, 1Kbyte, and 500byte. Each graph has
three parameters of comparison unit 1byte, 2byte, and 3byte.
The graphs in Figure 6 show the relation between signature size and two
parameters; signature extraction region and comparison unit. The more signature
extraction region and comparison unit increases, the more signature size increases.
Larger signature includes richer information about related non-self program. But,
when signature is used for scanning viruses, small signature is effective. Signatures
that are larger than 1KB are not feasible for a virus-scanning signature. Moreover, the
percentage of files that has zero signature is independent with size of signature
extraction region and comparison unit. The number of files with zero signature size is
almost 14 (8.75%) in all cases. We chose two effective sizes of signature extraction
region; 1Kbyte and 500 byte. We simulated comparison for signature selection about
the extracted signatures of non-self programs on these two parameters.

7. Artificial Immune System against Viral Attack 505
Similarity values between each signature of non-self programs are shown in Figure
7. When signature extraction region is 1Kbyte and comparison unit is 1byte, 88% of
signatures of non-self programs have similarity value zero. When signature extraction
region is 500byte and comparison unit is 1byte, signatures with zero similarity value
are 92% in entire signatures of non-self programs. Since the percentage of actual virus
infected file is 1.875%, ideal percentage of signatures that similarity value is zero
should be 98.125%. Then, we can classify non-self signatures into virus signatures,
whose similarity value is greater than zero. But, the percentage of non-zero signatures
is 6% even though the extraction region size is 1Kbyte and comparison unit size is 3
byte. We need to determine threshold value to classify non-self programs into normal
programs and viruses.
When SER is fixed, increasing the size of comparison unit results in rapid increase
of its similarity value, since consecutive extracted signature is larger. When larger
comparison unit is used, the gap of similarity value is larger. When signature
extraction region is 1Kbyte and comparison unit is 3byte, we can find easily threshold
of similarity value 1.E+08 to classify signatures of non-self programs into virus,
because ideal percentage of signatures of normal programs is 98.125%. When
threshold of similarity value is 1.E+08, three signatures are selected by signature
selector. Virus scanner using these signatures can detect virus-infected files.
(a) Percentage of Files as Similarity ( SER 1 Kbyte )
90%
92%
94%
96%
98%
100%
0.E+00 3.E+02 7.E+02 2.E+03 6.E+03 1.E+04 1.E+08
Similarity
PercentageofFiles
3 Byte
2 Byte
1 Byte
(b) Percentage of Files as Similarity ( SER 500 Byte )
90%
92%
94%
96%
98%
100%
0.E+00 3.E+02 7.E+02 2.E+03 6.E+03 1.E+04 1.E+08
Similarity
PercentageofFiles
3 Byte
2 Byte
1 Byte
Fig. 7. Percentage of files as similarity: In the case of SER 1Kbyte and 500byte, Percentage of
files is showed. Each graph has three parameters of comparison unit 1byte, 2byte, and 3byte.
5 Conclusion
Artificial immune system is based on distinguish “self” and “non-self” like biological
immune system. But previous artificial immune system is not feasible for dynamic
computer system, especially for negative selection algorithm. In this paper, we
proposed the artificial immune based Virus Detection System (VDS) that is feasible
for dynamic computer system. VDS is a signature learning system to detect unknown
virus. VDS produces signatures of non-self from suspicious program, and classify
them into self-program and virus. Since virus tends to infect other programs, if similar
non-self signatures are appeared frequently, VDS classifies those into signature of
viruses for virus scanning. Therefore, VDS detect adaptively unknown viruses
In the simulation of VDS with 1Kbyte of the Signature Extraction Region (SER)
and 3byte of the comparison unit, 94% of extracted signatures were completely

8. 506 H. Lee, W. Kim, and M. Hong
different. In other words, their similarity values are zero. The remaining 6%
signatures including virus signatures had distinguished similarity values. Especially,
the 2% virus signatures had relatively high similarity values. Therefore, proposed
VDS can classify suspicious non-self programs into normal programs and viral
programs. Using threshold of similarity value, VDS can select virus signatures. Virus
scanner using these signatures can detect virus-infected files correctly.
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